Researchers Using Breeding to Improve Cod's Resistance to Disease

NORWAY - It is important to minimise the risk of disease in cod aquaculture. Scientist Rama Bangera of Nofima has studied the effect that the cods genes have on its resistance to three fatal diseases, and found that the resistance of farmed cod can be increased by selective breeding.

Breeding is a highly effective tool in improving the production in farmed fish and thus the profitability of aquaculture. It was for this reason that a Norwegian breeding programme for cod was started in 2002. The programme is conducted by Nofima on commission from the Ministry of Trade, Industry and Fisheries, and with the goal of breeding cod suitable for farming with better growth performance than wild cod, and higher resistance against important fish diseases.

Extensive use of methods

In his doctoral work, Rama Bangera has investigated genetic aspects of disease resistance in cod, in collaboration with scientists at Nofima and the Norwegian University of Life Sciences (NMBU). He has investigated two diseases caused by bacteria (vibriosis and francisellosis) and one caused by nodavirus (viral nervous necrosis, VNN). He has analysed disease resistance using advanced statistical and molecular genetic tools in four studies.

In order to breed fish for increased disease resistance, the cod in the breeding programme have undergone challenge tests, in which they have been exposed to contagion from the bacteria and virus that cause vibriosis and VNN. The results from the challenge tests were used to identify families that have the highest resistance to disease. In conventional challenge tests, the ability of fish to resist disease is determined by recording whether the fish die or survive.

One of the advances that Mr Bangera describes in his doctoral work is the fact that he has studied not two groups (dead fish and survivors), but three. He can distinguish among the surviving fish between those infected and those not infected. The predictions of resistive ability are more reliable if the contagion status of the surviving fish is included in the model.

Resistance through breeding

Mr Bangera discovered a weak genetic correlation between growth and disease resistance against vibriosis and VNN. This means that it will be possible to achieve a genetic profile that is beneficial for both disease resistance and growth at the same time in the breeding programme.

Mr Bangera found regions in the genome that directly influence the genetic resistance to VNN. It will be possible to use these regions as markers in a technique known as “marker-assisted selection” (MAS). This will make it possible to select broodstock resistant to VNN, without exposing it to infection.

“Our broad test of various statistical methods shows how important it is to select the correct statistical method for the analysis of disease data, in order to determine the resistance of the fish more accurately and in this way achieve better breeding success for disease resistance,” said Mr Bangera.

Selection in the cod breeding programme was carried out for both growth and resistance to vibriosis.

The thesis concludes with the suggestion that future studies should look more closely at the effects of the selection methods used today, the practical use of MAS, and further identification of candidate genes for disease resistance in Atlantic cod.

Thesis defence

Rama Bangera is from India and works as scientist at Nofima in Tromsø. He had his public defence of his thesis on 2 October at NMBU in Ås.